Aspects and embodiments of the invention are directed to a photobioreactor and methods for producing a bioproduct using said photobioreactor; more particularly to a stacked, slab-type waveguide photobioreactor assembly and associated methods and applications thereof.
Concerns about the impact of climate change, CO2 emissions, and energy security have led to widespread interest in the production of biofuels from microalgae. Microalgae have higher CO2 fixation efficiencies and growth rates than other plant-based feedstocks and the potential to utilize waste-water or industrial gas wastes as nutrient sources. The most developed method for extracting biofuels from microalgae is converting their stored lipids into biodiesel, which utilizes a separation process that is very energy intensive. This has prompted the research and development of many engineered strains of cyanobacteria to directly secrete fuels such as hydrogen, ethanol, isobutyraldehyde, and other high value products.
To take advantage of these engineered strains, innovative photobioreactor (PBR) designs are required that can sustain high density cultures while enabling efficient light delivery and gas exchange. The most common reactors used in algal cultivation are open raceway ponds and tubular-type enclosed reactors. While these designs do have respective advantages, both are faced with fundamental limitations in delivery of sufficient light and CO2 and extraction of products to maintain high photosynthetic rates. Uneven light distribution causes the culture to be overexposed at the surface and underexposed below the light penetration depth. To counteract this problem many approaches have been investigated including the integration of optical fibers, interaction with evanescent and plasmonic fields, and planar waveguides.
In parallel to the problem of light delivery, limitations in gas exchange and transport are also being addressed. Traditionally, gas exchange in PBRs is provided by bubbling or passive exposure to the atmosphere. Though easy to implement, these methods constrain optimal PBR geometries and operation, and limit possible culture densities. CO2 delivery is limited by uneven distribution throughout the reactor volume. Maintaining a uniform distribution is important for efficient volume utilization since regions with low CO2 concentration suffer from lower rates of photosynthesis. Turbulent flow mixing is a common mechanism by which CO2 concentration is equilibrated. This condition requires that a large amount of energy is spent on mixing the algal cultures, up to 41% of the cultivation energy budget in some cases, and contributes to the already high energy costs of the algal cultivation process. This has motivated research into various methods for improving gas exchange coefficients and reducing the mixing energy demands in PBRs.
A recent advance in enhancing gas exchange has been the integration of hollow fiber membranes (HFMs) modules into PBRs. A HFM consists of hollow fibers with membranes that allow for gas exchange between the media inside and outside the fiber. HFMs have been used in the chemical, petrochemical, pharmaceutical and galvanic industries, and applied in such varied applications as wastewater treatment, drinking water treatment, tissue engineering, and the development of artificial organs. Recently, HFMs have shown potential to address the gas exchange challenges faced by PBRs and several lab scale reactors incorporating HFM modules have been reported. These studies have verified the potential benefits of integration of HFMs into PBRs by reporting increased biomass production, improved gas exchange, regulation of pH, and promotion of CO2 fixation.
In view of the challenges and problems appreciated in the photobioreactor art and with the efficient production of biofuels using photobioreactors, the inventors have recognized the advantages and benefits to be realized by addressing these challenges and solving these problems, many of which are realized by the embodied invention directed to high density photobiorefineries with optimized light/CO2 delivery and product extraction, as disclosed and claimed herein below.